Electric ducted fan

ABSTRACT

An electric ducted fan for an aircraft is shown. A nacelle defines a duct that houses a propulsive fan having a fan diameter DF. An electric machine is configured to drive the fan, and has an electromagnetically active length LA and an electromagnetically active diameter DA defining an aspect ratio (LA/DA) of from 0.8 to 2. A speed reduction device is located between the electric machine and the fan, and has a reduction ratio of at least 3:1. A ratio of the electromagnetically active diameter DA to the fan diameter DF (DA/DF) is from 0.3 to 0.5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from British Application No. 1807770.1 filed on May 14, 2018 the entire contents of which are incorporated by reference.

BACKGROUND Technical Field

This disclosure relates to configurations of electric ducted fans for aircraft.

Description of the Related Art

Ducted fan-type propulsors are used for a large proportion of aircraft due to their greater efficiency and reduced noise due to lower tip losses than open propellers. Turbofans utilise a gas turbine core to drive a fan, the fan being larger than the core to produce a bypass flow responsible for the majority of thrust.

Despite great advances in materials, compressor and turbine aerodynamics, and combustion efficiency, the gas turbine engines used in the cores of turbofans are still quite thermally inefficient. In particular, gas turbines are less efficient the smaller they are, which restricts possibilities in terms of the number of engines that may be installed on an airframe. Their fuel source is also not renewable. Further, the jet of high enthalpy exhaust from the core is responsible for a large amount of noise when it mixes with the exhausted bypass flow.

It is therefore desirable to utilise electric machines in place of gas turbine engines in ducted fan arrangements to alleviate some or all of the aforesaid issues.

SUMMARY

The present disclosure is directed towards electric ducted fans for aircraft.

One such electric ducted fan comprises a nacelle defining a duct, and a propulsive fan in the duct having a fan diameter D_(F). An electric machine is configured to drive the fan via a speed reduction device having a reduction ratio of at least 3:1. The electric machine has an electromagnetically active length L_(A) and an electromagnetically active diameter D_(A) defining an aspect ratio (L_(A)/D_(A)) of from 0.8 to 2. A ratio of the electromagnetically active diameter D_(A) to the fan diameter D_(F) (D_(A)/D_(F)) is from 0.3 to 0.5.

Electric ducted fans constructed in such a manner have various advantages, such as reduced nacelle drag, a shorter pylon, and increased ground clearance.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which:

FIG. 1 shows an electric ducted fan propulsor which uses an electric machine to drive a fan via a speed reduction device;

FIG. 2 shows definitions of various geometric parameters;

FIG. 3A shows the propulsor of FIG. 1, using a star-type gearbox as the speed reduction device;

FIG. 3B shows the propulsor of FIG. 1, using a planetary-type gearbox as the speed reduction device;

FIG. 4 shows one configuration of the electric machine; and

FIG. 5 shows another configuration of the electric machine.

DETAILED DESCRIPTION

An electric ducted fan propulsor is shown in FIG. 1.

The propulsor is shown generally at 101, attached to a wing 102 of an aircraft (not shown) by a pylon 103.

Being a ducted fan, the propulsor 101 comprises a nacelle 104 which defines a duct 105 having an inlet 106 and a nozzle 107, and in which a propulsive fan 108 is located. In operation, the fan 108 raises the pressure of intake air, with swirl in the airflow being removed by outlet guide vanes 109. The airflow is directed through the nozzle 107 to generate thrust.

In the embodiment of FIG. 1, the fan 108 is driven by an electric machine 110, in combination with a speed reduction device 111.

In the present embodiment, the electric machine 110 is rated at a maximum continuous power of between 100 kilowatts and 100 megawatts. In a specific embodiment, the electric machine 110 is rated at a maximum continuous power of between 1 megawatt and 10 megawatts. In a more specific embodiment, the electric machine 110 is rated at 2 megawatts maximum continuous.

In the present example, the speed reduction device 111 is an epicyclic gearbox, but could be another form of reduction device such as a layshaft-based gear system or other suitable design. In one example, the epicyclic gearbox is a star gear system. Such an arrangement will be described with reference to FIG. 3A. In another example, the epicyclic gearbox is a planetary gear system. Such an arrangement will be described with reference to FIG. 3B.

The use of speed reduction device allows 111 allows the electric machine to operate at higher speed, and the fan to operate at lower speed. In this way, both components operate in their respective regimes of higher efficiency. Further, a larger fan is permitted due to the reduction of tip speed, and a smaller electric machine is permitted.

Thus, as illustrated in FIG. 2, the arrangement of FIG. 1 permits a combination of a fan having a high hub-tip ratio, and a high aspect ratio electric machine, in terms of its length-to-diameter.

The hub-tip ratio of the fan 108 in the present example is the ratio of the diameter D_(F) of the leading edge of the fan blades 201, to the diameter of the diameter D_(H) of the hub 202 at the leading edge 203 of the fan blades 201, i.e. the diameter of the inner gas-washed surface of the fan 108. It will be appreciated that the hub-tip ratio is a standard, well-known property of a fan, compressor, or turbine stage.

The length of the electric machine as defined herein is the maximum length L_(A) of the electromagnetically active components, whilst the diameter of the electric machine as defined herein is the maximum diameter D_(A) of the electromagnetically active components. Examples of the definitions of length L_(A) and diameter D_(A) as applied to specific machine types will be described further with reference to FIGS. 4 and 5.

Benefits are obtained by specifying the following parameters for the propulsor 101:

(i) the aspect ratio of electric machine 110, L_(A)/D_(A) (i.e. the value of L_(A) divided by D_(A)), being from 0.8 to 2;

(ii) the reduction ratio of the speed reduction device being at least 3:1;

(iii) the ratio of the diameters of the electromagnetically active components in the electric machine and the fan, D_(A)/D_(F) (i.e. the value of D_(A) divided by D_(F)), being from 0.3 to 0.5.

In particular, the inventor has discovered that this combination of values advantageously enables the propulsor 101 to have a smaller diameter nozzle 107 for the same overall fan pressure ratio, which allows the pylon 103 to be made shorter. This results in, for a particular fan diameter D_(F), greater ground clearance, or a greater fan diameter D_(F) for a particular ground clearance. Further, the reduction in pylon length and height results in a reduced moment on the wing 102, and a reduction in weight.

In a specific embodiment, the ratio L_(A)/D_(A) is from 1.1 to 1.7. The inventor has discovered that this allows a narrowed nozzle, along with a higher speed machine. In another specific embodiment, L_(A)/D_(A) is from 1.3 to 1.5. In a more specific embodiment, L_(A)/D_(A) is 1.4.

In an additional or an alternative embodiment, the ratio D_(A)/D_(F) is from 0.35 to 0.45. In a specific embodiment, the ratio D_(A)/D_(F) is from 0.37 to 0.43. In a more specific embodiment, the ratio D_(A)/D_(F) may be 0.4.

The hub-tip ratio of the fan 108, i.e. the value of D_(T)/D_(H), may be from 0.25 to 0.31. In a specific embodiment, the hub-tip ratio may be from 0.27 to 0.29. In a more specific embodiment, the hub-tip ratio may be 0.28.

In an embodiment, the fan 108 has a tip pressure ratio (i.e. the ratio of the stagnation pressure immediately upstream of the tip of a fan blade 201, and immediately downstream of the fan blade 201) of from 1.3 to 1.7 at an altitude of 35,000 feet above sea level and a temperature of minus 54 degrees Celsius (i.e. ISA+0 standard conditions), and a true airspeed of Mach 0.85, i.e. during cruise conditions. In a specific embodiment, the tip pressure ratio may be from 1.4 to 1.6 in the aforesaid conditions. In a more specific embodiment, the tip pressure ratio may be 1.5 in the aforesaid conditions. In another specific embodiment, the tip pressure ratio may be 1.38 in the aforesaid conditions.

A diagram of one embodiment of the propulsor 101 is shown in FIG. 3A, in which the speed reduction device 111 is a star-type epicyclic gearbox, indicated generally at 301. The electric machine 110 is coupled via an input shaft to a sun gear 302, which meshes with planet gears 303 mounted to a static carrier 304. The planet gears 303 transfer torque to a ring gear 305, which drives the fan 108. In an embodiment, the reduction ratio of the star gearbox 301 is from 3:1 to 3.7:1. Use of a star gearbox may assist in terms of reducing complexity of installation due to the fixed carrier, for example with respect to lubrication systems.

A diagram of another embodiment of the propulsor 101 is shown in FIG. 3B, in which the speed reduction device 111 is a planetary-type epicyclic gearbox, indicated generally at 311. The electric machine 110 is coupled via an input shaft to a sun gear 312, which meshes with planet gears 313 mounted in a rotating carrier 314. The planet gears 313 transfer torque to a static ring gear 315, with the carrier driving the fan 108. In an embodiment, the reduction ratio of the planetary gearbox 311 is from 3:1 to 4.7:1. Use of a planetary gearbox may assist in terms of providing a greater reduction ratio for a given number of gear teeth.

An example of a configuration of the electric machine 101 is shown in cross-section through its central axis A-A in FIG. 4. This particular configuration is a radial flux electric machine 401, which comprises a stator 402 which surrounds a rotor 403. The stator 402 comprises a lamination stack of the known type. In the present example, the electric machine is a permanent-magnet machine and thus the rotor 403 comprises permanent magnets, which interact with the magnetic field generated by windings in the stator 402 to generate torque. Alternative machine types such as induction machines may also be employed. As a radial flux machine, end windings 404 emerge axially from the lamination stack of the stator 402.

As described previously, the electric machine 401 may be described as having an electromagnetically active length L_(A) and an electromagnetically active diameter D_(A). As used herein, “electromagnetically active” refers to the region responsible for generating torque upon the rotor. Thus in the present example, the length L_(A) is the length of the lamination stack of the stator 402, and does not include the end windings 404 as they do not generate an appreciable torque upon the rotor 403. Similarly, the diameter D_(A) is the diameter of the lamination stack of the stator 302.

Put another way, the “electromagnetically active” parts of the electric machine 401 may be characterised as those components which form part of the torque-generating magnetic circuit in the machine.

Those skilled in the art will appreciate how this definition may read across to other machine types such electromagnet-based synchronous motors, and axial flux machines, with the definitions of aspect ratio as set out herein also applying to these types of machine.

Another example configuration of the electric machine 101 is shown in cross-section through its central axis B-B in FIG. 5. This particular configuration is a radial flux electric machine 501, which comprises a rotor 502 which surrounds a stator 503. As with electric machine 401, the electromagnetically active length L_(A) of this machine is the length of the lamination stack of the stator 503, not including the end windings 504. The electromagnetically active diameter D_(A) is, however, in this embodiment, the diameter of the rotor 502. Again, machine 501 is a permanent-magnet machine, and thus the diameter D_(A) is bounded by the greatest radial extent of the magnets therein. Should the machine instead be an induction machine, the diameter D_(A) would be defined by the greatest radial extent of the rotor iron.

More generally, as described with reference to FIG. 4, it will be appreciated the “electromagnetically active” parts of the electric machine 501 may be characterised as those components which form part of the torque-generating magnetic circuit in the machine.

Various examples have been described, each of which feature various combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein. 

The invention claimed is:
 1. An electric ducted fan for an aircraft, comprising: a nacelle defining a duct; a propulsive fan in the duct having a fan diameter D_(F); an electric machine configured to drive the fan, the electric machine having an electromagnetically active length L_(A) and an electromagnetically active diameter D_(A) defining an aspect ratio (L_(A)/D_(A)) of from 0.8 to 2; a speed reduction device between the electric machine and the fan, having a reduction ratio of at least 3:1; and wherein a ratio of the electromagnetically active diameter D_(A) to the fan diameter D_(F) (D_(A)/D_(F)) is from 0.3 to 0.5.
 2. The electric ducted fan of claim 1, in which the aspect ratio (L_(A)/D_(A))/is from 1.1 to 1.7.
 3. The electric ducted fan of claim 2, in which the aspect ratio (L_(A)/D_(A))/is 1.4.
 4. The electric ducted fan of claim 1, in which the ratio of the electromagnetically active diameter D_(A) to the fan diameter D_(F) (D_(A)/D_(F))/is from 0.35 to 0.45.
 5. The electric ducted fan of claim 4, in which the ratio of the electromagnetically active diameter D_(A) to the fan diameter D_(F) (D_(A)/D_(F))/is from 0.37 to 0.43, for example 0.4.
 6. The electric ducted fan of claim 1, in which the electric machine is a radial flux electric machine.
 7. The electric ducted fan of claim 6, in which the stator is exterior to the rotor, the electromagnetically active diameter D_(A) is the diameter of a stator stack, and the electromagnetically active length L_(A) is the length of the stator stack.
 8. The electric ducted fan of claim 6, in which the stator is interior to the rotor, the electromagnetically active diameter D_(A) is the diameter of the rotor, and the electromagnetically active length L_(A) is the length of the stator stack.
 9. The electric ducted fan of claim 1, in which the fan has a hub-tip ratio of from 0.25 to 0.31.
 10. The electric ducted fan of claim 9, in which the fan has a hub-tip ratio of from 0.27 to 0.29, for example 0.28.
 11. The electric ducted fan of claim 1, in which the propulsive fan has a tip pressure ratio of from 1.3 to 1.7 at an altitude of 35,000 feet and a true airspeed of Mach 0.85.
 12. The electric ducted fan of claim 11, in which the propulsive fan has a tip pressure ratio of from 1.3 to 1.5 at an altitude of 35,000 feet and a true airspeed of Mach 0.85.
 13. The electric ducted fan of claim 12, in which the propulsive fan has a tip pressure ratio of 1.38 at an altitude of 35,000 feet and a true airspeed of Mach 0.85.
 14. The electric ducted fan of claim 1, in which the speed reduction device is a star gearbox with a reduction ratio of at most 3.7:1.
 15. The electric ducted fan of claim 1, in which the speed reduction device is a planetary gearbox with a reduction ratio of at most 4.7:1. 